What key feature differentiates X.509 certificates?

The distinguishing feature of X.509 certificates is that they are signed using a trusted Certificate Authority (CA). This signature by a CA establishes a chain of trust, allowing users and systems to verify the authenticity of the certificate. The CA confirms the identity of the entity that holds the certificate, ensuring that the public key it contains indeed belongs to that entity. This process is critical in supporting secure communications over networks, enabling services like HTTPS for secure web browsing.

Other options are not applicable to X.509 certificates: they are not limited to symmetric encryption, as they primarily support asymmetric encryption using the public keys they contain. They include public keys, essential for the encryption and authentication processes, and they are not subject to time constraints in their usage. Thus, the reliance on a trusted CA for signing is what fundamentally differentiates X.509 certificates in the context of digital security.